[Asterisk-Dev] Re: Smart Asterisk.

[Mark Spencer]

> 1. If UA's source IP NOT equal to Contact Header field's IP or UA's
> SIP request SDP's Connection IP, then the phone is behind the NAT.

Unfortunately that's not actually accurate.  Believe it or not there are
many situations (e.g. pingtel) where the Contact *is* different from the
source and if you try to respond to the source, your packets will be
dropped on the floor.

This is a design flaw in SIP, so strictly speaking NAT functionality is
incompatible with strict RFC2833 interpretation.

Mark